Nonredox trivalent nickel catalyzing nucleophilic electrooxidation of organics

A thorough comprehension of the mechanism behind organic electrooxidation is crucial for the development of efficient energy conversion technology. Here, we find that trivalent nickel is capable of oxidizing organics through a nucleophilic attack and electron transfer via a nonredox process. This nonredox trivalent nickel exhibits exceptional kinetic efficiency in oxidizing organics that possess the highest occupied molecular orbital energy levels ranging from −7.4 to −6 eV (vs. Vacuum level) and the dual local softness values of nucleophilic atoms in nucleophilic functional groups, such as hydroxyls (methanol, ethanol, benzyl alcohol), carbonyls (formamide, urea, formaldehyde, glucose, and N-acetyl glucosamine), and aminos (benzylamine), ranging from −0.65 to −0.15. The rapid electrooxidation kinetics can be attributed to the isoenergetic channels created by the nucleophilic attack and the nonredox electron transfer via the unoccupied eg orbitals of trivalent nickel (t2g6eg1). Our findings are valuable in identifying kinetically fast organic electrooxidation on nonredox catalysts for efficient energy conversions.

1.I would recommend the authors to corroborate the existence of Ni3+ species during UOR with Xray absorption measurements as well if possible.
2. The EPR data illustrated in Figure S12 are too weak to obtain reliable quantitative analysis of the NiOH2 electrode before and after UOR.The error bars have further not been provided.This data set needs to be repeated before the manuscript can be publishable.
3. The following sentences are difficult to read and not well constructed.Please correct: a. "To circulate the electrochemical oxidation and chemical regeneration of Ni(OH)2 will achieve a decoupled hydrogen production."b. "And the room temperature chemical reaction between Ni3+ and organics is a kinetically rapid…." Reviewer #3 (Remarks to the Author): This manuscript describes the electrooxidation of organic compounds by Ni(OH)2.It is found that in contrast to water oxidation, the reaction occurs already at the NiOOH state.A clear correlation between the HOMO energy of the substrate and/or the softness of the coordinating atom of the substrate with the tendency to get oxidized at the Ni(III) state.The findings are very interesting and it seems clear that the active state is NiOOH.The correlation with the molecular descriptors gives easy predictability and should be of interest to a broader community.My assessment of the computations is that they were performed at a satisfactory level.They are quite simple but contain the necessary information to predict the reactivity.One small remark is that the calculations were performed at different levels of theory for the small organics and the solid, and perhaps some care should be taken when comparing energy levels.However, I believe that the conclusion would be identical if both systems were calculated at the same level, except that the numbers would shift slightly.Overall I believe that this is a very interesting manuscript with well founded conclusions in a field that in really emerging at the moment.I recommend that it is published as is.
Reviewer #4 (Remarks to the Author): Yan S. and coll.investigate in this paper the mechanism of electrocatalytic urea oxidation at Ni(OH)2 electrodes.In their study, the authors support that this mechanism shuttles via non-redox processes leaving the oxidation state of Ni(+III) untouched during catalysis.The study combines computational calculations to assess the softness and HOMO energy levels of the substrate and surface sites, electrochemical measurements of i/E curves, open-circuit potentials, coupling to Raman spectroscopy, and impedance spectroscopy.
The quality of the data is of overall good level and the presentation of this data is well done.However, I find that the hypothesis brought by the authors, that UOR proceeds through Ni centers that remain in their +III oxidation state, is not sufficiently strongly supported.
I develop here my points: -First, discussing catalysis requires that a catalytic outcome is actually probed.This report does not contain results of electrocatalytic outcome (product quantification, selectivity, faradaic efficiency) for UOR with the electrodes used by the authors, nor an obvious reference to the same system.Without that, discussing mechanistic considerations seems vain.
-OCP and Raman experiments (Fig. 2d, e) show that Ni(III) species can actually oxidize urea.
-It is very obscure to me why the authors recoursed to cell voltage measurements to estimate kinetics at the anode.This procedure is more likely to introduce kinetics at the cathode in the whole problem.Similar steady-state OCP values for voltages vs ref and vs CE does not mean that the transient kinetics at the cathode can be ignored.I thus do not see how the authors conclude that the cell voltage depends on Ni2+/3+ kinetics.In addition, it was already proposed above that this kinetics is slow, likely due to phase change (hysteresis in LSV).
-The experiments with chelation by a dmg salt are not conclusive.Under catalytic conditions, the potential applied is always more positive than E(Ni(III/II)) and thus formed Ni(II) species short-lived.When CV cycling, it is not the case, since potential excursions below E(Ni(III/II)) quantitatively convert the species into Ni(II) over extended time.At most, these comparative experiments allow to conclude that there is no long-lived soluble Ni(II) intermediate in UOR and are by far not a "solid evidence [] that urea oxidation does not undergo the Ni2+/Ni3+ redox couple".

Some secondary issues:
-Fig 4d .IR: C-N and C-O vibrations are almost baseline.Also, is there rationale why these vibrations are observed but no N-H ones?-Sup.Fig. 13: the UOR equation is not equilibrated (O).I would at this pH write it with H2O, not H+.And thus the Nernst equation should also be corrected.
-An "onset" potential has no meaning.One should better report the potential at a given current density.
-The text is often very hard to read and understand.An important style-editing would be needed before publication.
In conclusion, the main claim of the authors does not appear to be backed enough by data and there is at this stage no solid experimental evidence that sustains the mechanistic hypothesis.In my opinion, the strongest claim that can currently be made is that Ni(II) are not long-lived intermediates (/resting states), which is however not surprising given the applied potentials.If the authors wish to to further address the implication of the Ni(III) and Ni(II) species in UOR, I would suggest performing additional experiments, such a with operando spectroscopies that enable addressing oxidations state (XAS, XPS,…).On these grounds, I have selected reject and resubmit.

Response to Reviewer's comments (NCOMMS-23-34704A)
Reviewer #1 (Remarks to the Author): Yan et al report nonredox trivalent nickel catalyzing nucleophilic electrooxidation where clearly show that the electrooxidation of organics on Ni 3+ species does not follow the Ni 2+/3+ redoxmediated electron transfer mechanism.Both theoretical and experimental techniques were characterized to verify above viewpoint.This work is very interesting for the researchers studying the nickel electrocatalysis, and the discovery seems solid and credible.Therefore, I recommend this work published in Nature Communications after minor revisions below: 1.A scheme for this work, including materials, theoretical and experimental techniques, must be represented in a one scheme for emphasizing the highlight point, thought and whole process for texting, in order to improve the readability.
Reply: Thanks for your valuable suggestions for improving the readability of our study.A scheme of our work was provided as Fig. 1c in the main text.2. In Figure 3f, it seems that the electron transfer mechanism in Ni-O-Ni configuration like the double-exchange/super-exchange effect in Mn-O-Mn system, what is the differences between them?
Reply: Thanks for your deep thinking on the electron transfer mechanism.The XAS data clearly show that single Ni 3+ species exist in a distorted NiO6 octahedron of NiOOH (J.Electrochem. Soc., 1996, 143, 1613-1616).And in our study, we have demonstrated by various methods including In situ XANES, in situ UV-vis absorption spectra, and in situ Raman spectra, that the UOR is catalyzed by Ni 3+ in NiOOH.The double exchange mechanism, an exchange interaction between two metal ions with parallel alignment of the spins separated by an oxygen ion, was first found in the Mn 3+ -O-Mn 3+ (or Mn 4+ ) by Zener in 1951( Phys. Rev. 1951, 82, 403-405).According to the ligand field theory, a Jahn-Teller distortion of octahedrally coordinated NiO6 units in NiOOH suggests that the Ni species in NiOOH is low-spin 3d 7 Ni 3+ with the t2g 6 eg 1 electronic configuration (Adv. Funct. Mater. 2022, 32, 2111234;Adv. Mater. 2023, 35, 2203420), thus following a double exchange mechanism, the same as the Mn-O-Mn.
3. It should be added DTG curves in Figure 4b, and corresponding DSC test should be added to further verify the opinion.
Reply: Thanks for your help.We have updated the data in Figure 4b.The TGA, DTG, and DSC curves were added to the Fig. 4b. 4.There is some inconsistence in Electrochemical details.For example, in "electrochemical measurement" part, ; Meanwhile, in "In situ UV-vis absorption measurements" part, CHI660E and Ag/AgCl as reference.Why aren't their testing conditions as consistent as possible?
Reply: Thanks for carefully reading our study.Indeed, in our study, most electrochemical measurements, including LSV, CV, and OCP, were carried out on CHI 660 and used Hg/HgO as reference.However, for the in situ UV-vis absorption measurement, the reference electrode used is limited by the size of the in situ reaction cell.A cuboid-shaped quartz cuvette, with interior dimensions of 10 mm (width) and 10 mm (length), was used as the in situ reaction cell.So, the Ag/AgCl with a diameter of 4 mm was able to be input into the cell as a reference electrode compared Hg/HgO electrode with a diameter of 6 mm.That is, the Ag/AgCl reference electrode used in this in situ measurement is to ensure that all the electrodes including reference electrode, working electrode, and counter electrode can be put into the small quartz cuvette.The work by Yan, Zou and co-workers aims at mapping the water and organics oxidation reactions catalyzed by trivalent Ni-based catalysts, and addressing the differences in their mechanisms that are currently under heavy debate.An array of characterization techniques are employed to map the oxygen evolution reactions and urea oxidation reactions on Ni(OH)2 electrodes.Proof of the UOR reactions driven by Ni 3+ electron transfer kinetics for the Ni 3+ /urea oxidation reactions are further provided.The manuscript is very well written, and the experiments have been thoroughly conducted.I recommendation publication of this work in Nature Communications after the following comments are addressed: 1.I would recommend the authors to corroborate the existence of Ni 3+ species during UOR with X-ray absorption measurements as well if possible.
Reply: Thanks for your help.The X-ray absorption near edge structure (XANES) spectra were carried out and provided in Figure 3d.The photon energy was calibrated by the first peak maximum of the first derivative of a nickel foil (8333.0eV).The X-ray absorption near edge structure (XANES) spectra revealed that the intensity of the white line decreases with polarizing the Ni(OH)2 electrode from 1.3 to 1.5 V in 1 M KOH + 0.33 M urea, suggesting the formation of higher-valence Ni species due to that the high-valence Ni inducing distortion in the NiO6 octahedral configuration will result in a decrease in the white line intensity (J.Electrochem.Soc. 1990,137, 383-388).Meanwhile, the edge energy (measured at half height) for the electrode at 1.3 and 1.5 V is the same as those of the Ni(OH)2 (8342.6 eV) and NiOOH (8344.5 eV), respectively.This edge position suggests a +3 nickel oxidation state for the electrode during UOR (J.Am.Chem. Soc. 2012, 134, 6801-6809).The in situ XANES results confirmed that during UOR the Ni 3+ is UOR-active and is likely to keep a constant valence state, in good agreement with the results from in situ Raman and in situ UV-vis absorption.the NiOH2 electrode before and after UOR.The error bars have further not been provided.This data set needs to be repeated before the manuscript can be publishable.
Reply: Thanks for your help.To strengthen the EPR signals, the EPR data were updated on an advanced Bruker (EMXplus X-band, Germany) electron paramagnetic resonance spectrometer.The error bars were provided based on the statistical data from three samples.Reply: Thanks for your help.We have revised these sentences as follows: a. Accordingly, we can divide the process into two steps: an electrochemical step that reduces water at the cathode to produce hydrogen and oxidizes the anode to form NiOOH in 1 M KOH electrolyte, followed by a spontaneous chemical step reduces the anode back to its initial state by oxidizing organics.The spatially separated two-step processes will achieve hydrogen production and oxidation of organics in different reaction chambers, thus benefiting to produce the high-purity products.
b.And the spontaneous reaction of Ni 3+ oxidizing organics is kinetically rapid at room temperature, suggesting that this technique to couple with hydrogen production is low-cost and time-effective.

Reviewer #3 (Remarks to the Author):
This manuscript describes the electrooxidation of organic compounds by Ni(OH)2.It is found that in contrast to water oxidation, the reaction occurs already at the NiOOH state.A clear correlation between the HOMO energy of the substrate and/or the softness of the coordinating atom of the substrate with the tendency to get oxidized at the Ni(III) state.The findings are very interesting and it seems clear that the active state is NiOOH.The correlation with the molecular descriptors gives easy predictability and should be of interest to a broader community.My assessment of the computations is that they were performed at a satisfactory level.They are quite simple but contain the necessary information to predict the reactivity.One small remark is that the calculations were performed at different levels of theory for the small organics and the solid, and perhaps some care should be taken when comparing energy levels.However, I believe that the conclusion would be identical if both systems were calculated at the same level, except that the numbers would shift slightly.Overall I believe that this is a very interesting manuscript with well founded conclusions in a field that in really emerging at the moment.I recommend that it is published as is.
Reply: Thank you very much for your affirmation and approval to our research.Indeed, as pointed out by the Reviewer, the DFT calculations for Fermi level of NiOOH and HOMO level of organics were performed by different methods on the DMol 3 software package and Gaussian 09W, respectively.For comparing energy levels, all the energy levels were referenced to the vacuum level.

Reviewer #4 (Remarks to the Author):
Yan S. and coll.investigate in this paper the mechanism of electrocatalytic urea oxidation at Ni(OH)2 electrodes.In their study, the authors support that this mechanism shuttles via nonredox processes leaving the oxidation state of Ni (+III) untouched during catalysis.The study combines computational calculations to assess the softness and HOMO energy levels of the substrate and surface sites, electrochemical measurements of i/E curves, open-circuit potentials, coupling to Raman spectroscopy, and impedance spectroscopy.The quality of the data is of overall good level and the presentation of this data is well done.However, I find that the hypothesis brought by the authors, that UOR proceeds through Ni centers that remain in their +III oxidation state, is not sufficiently strongly supported.
I develop here my points: -First, discussing catalysis requires that a catalytic outcome is actually probed.This report does not contain results of electrocatalytic outcome (product quantification, selectivity, faradaic efficiency) for UOR with the electrodes used by the authors, nor an obvious reference to the same system.Without that, discussing mechanistic considerations seems vain.
Reply: Thank you very much for your help.To confirm that the UOR on Ni(OH)2 electrode is a catalytic reaction, we have checked the product quantification, selectivity, and faradaic efficiency.The electrooxidation of urea follows a reaction of CO(NH2)2 + 6OH -→ N2 + CO2 + 5H2O + 6e -.This means that the UOR products are the gaseous N2 and CO2 and liquid H2O.Considering that both the H2O and CO2 are soluble in 1 M KOH, the product, N2, was analyzed by gas chromatography (GC) to confirm the selectivity and faradaic efficiency of UOR.The Faraday efficiency for N2 is 94.5 % (near 100%, the deviation mainly resulted from the N2 dissolving in electrolyte) at potentials above 1.4 V as shown in Supplementary Fig. 12a.The electron transfer amounts for N2 generation were equal to the amount of electric charge that passed through the electrode during UOR, suggesting a UOR catalytic reaction to occur.Furthermore, the selectivity of UOR was confirmed by the 13 C NMR spectra to detect the liquid products of long-time UOR.As shown in Supplementary Fig. 12b, no liquid products were detected after UOR, suggesting the high product selectivity for overall urea oxidation to CO2, N2, and H2O.
Supplementary Figure 12 | a, The N2 faradaic efficiency at anodic potentials between 1.35 and 1.5 V. b, The 13 C NMR spectra to identify the UOR product.
-OCP and Raman experiments (Fig. 2d, e) show that Ni(III) species can actually oxidize urea.
-It is very obscure to me why the authors recoursed to cell voltage measurements to estimate kinetics at the anode.This procedure is more likely to introduce kinetics at the cathode in the whole problem.Similar steady-state OCP values for voltages vs ref and vs CE does not mean that the transient kinetics at the cathode can be ignored.I thus do not see how the authors conclude that the cell voltage depends on Ni 2+/3+ kinetics.In addition, it was already proposed above that this kinetics is slow, likely due to phase change (hysteresis in LSV).
Reply: Thank you very much for your help.The LSV curve told us the occurrence of phase transition of Ni(OH)2 to NiOOH, which includes a coupled information of both thermodynamic and kinetic processes.In particular, during UOR, the kinetics for phase transition of Ni(OH)2 to NiOOH is completely coupled with the UOR kinetics.Therefore, we carried out the cell voltage measurements to further estimate kinetics at the anode.Indeed, as pointed out by the Reviewer, this procedure is possible to introduce kinetics at the cathode.Here, we used the Pt electrode as cathode to minimize the effects of cathodic kinetics on cell voltage.To clearly demonstrate the fast kinetics of high-activity Pt electrode, we use the Pt foil as both the anode and the cathode and monitor the cell voltage when periodically altering the anodic potentials between 1.6 and 1.65 V, a potential window for UOR occurring on Pt electrode.As shown in Supplementary Figs.7b and 7c, no visible cell voltage decay can be observed when periodically altering the anodic potentials, confirming that the possible electrochemical processes on Pt are kinetically rapid.Accordingly, in our case of Ni(OH)2 anode and Pt cathode, the cell voltage decay is totally resulting from the sluggish kinetics of the anode.
Similarly, as for OCP measurement, the voltage decay of the anode was monitored in a three-electrode system, that is, a reference electrode, Hg/HgO, was used to monitor the voltage decay of the anode and the Pt foil was used as the cathode.In the three-electrode testing system, the electrochemical processes on the Pt electrode are kinetically rapid, as shown in Supplementary Figs.7b and 7c.The transient kinetics at the Pt cathode is rapid and thus its effects on kinetics of the anode can be ignored.7 | a, Cell voltage decay of Ptanode-Ptcathode after periodically altering the anodic potentials between 1.6 and 1.65 V with a stay time of 10 s at every potential point.b, The time-dependent cell voltage equilibrium of Ptanode-Ptcathode when periodically altering the anodic potentials between 1.6 and 1.65 V.

Supplementary Figure
Abruptly altering the anodic potentials between 1.6 and 1.65 V on the Pt foil, the cell voltage immediately reaches the equilibrium state, indicating fast transient kinetics on both cathode of Pt foil and anode of Pt foil, to rapidly adjust the Fermi level of the electrode, thus sensitively adapting to the potential changes.These facts confirmed that the cell voltage is mainly dominated by the anodic potentials.Reply: Thank you very much for your help.In our study, the experimental evidence chains were focused on ruling out a possibility of electron transfer during UOR via Ni(OH)2/NiOOH, a phase transformation process.So, in order to avoid the possible confusing, we revised the "Ni 2+ /Ni 3+ redox couple" as "Ni(OH)2/NiOOH redox couple" in whole main text.
In addition, the main worry from the Reviewer is if there is a short-lived Ni(II) species to produce during UOR on Ni 3+ active species.To discuss this possibility, we need to distinguish the resting state and working state for a given electrochemical reaction.Here, the working state means that there is a steady current flowing through the electrode, and resting state describes a steady sate without electron transfer.Indeed, as pointed out by the Reviewer, under catalytic conditions, a working state for catalyst, the potential applied is always more positive than E(Ni(III/II)) to maintain the Ni 3+ states.No complexing reaction occurs on NiOOH at the working state.However, we switched the working state to a resting state, that is, this electrode is under open-circuit conditions, the complexing reaction was occurring after spontaneous chemical reduction of NiOOH by urea to Ni(OH)2 (Supplementary Fig. 9).This fact means that the dimethylglyoxime can chelate Ni 2+ in the amorphous and defect-rich restingstate Ni(OH)2.Further, we carried out the CV cycling to simulate the working-state Ni(OH)2.And, during the CV cycling with a wide potential window from 0.9 to 1.3 V for Ni(OH)2 generation, the complexing reaction occurs and completely etches the Ni(OH)2 away from the electrode.These evidences suggested no occurrence of Ni(OH)2 during UOR on NiOOH.This is a truly key point, in our study, if there are short-lived Ni(II) species during UOR on NiOOH.To resolve this doubt, we would need to look back on the electron transfer mechanism of electrochemical reaction.In 1931, a quantum-mechanical theory of electron transfer at the electrode/electrolyte interface was originated by R. W. Gurney (Proceedings of the Royal Society of London.Series A, Containing Papers of a Mathematical and Physical Character, 1931, 134, 137-154).According to this theory, the steady current flowing through the electrode is a result of electron transitions between levels of equal energy at the electrode/electrolyte interface via tunneling the interfacial barriers.Subsequently, in 1952, W. F. Libby (J.Phy.Chem. 1952, 56, 863) introduced the Franck-Condon principle to describe the electrochemical electron transfer.The isoenergetic tunneling of electrons has been proved to occur when reducing and oxidizing ions or molecules satisfy the requirement of the Franck-Condon principle.An electron transfer in electrochemical reaction is like a charge-transfer transition in electronic spectroscopy.It occurs in a very short time and the various nuclei can be thought of as fixed in position during the transition (the Franck-Condon principle).Libby pointed out that when an electron is transferred from one ion or molecule to another, the "jump" is instantaneous and the nuclear environment around each ion does not have time to change during the jump itself (the Franck-Condon principle).
In 1955, R. A. Marcus developed an electron transfer theory, called Marcus theory (J.Chem.Phys. 1956, 24, 966;J. Chem. Phys. 1965, 43, 679;Ann. Rev. Phys. Chem. 1964, 15, 155).In Marcus theory, the electron transfers in ordinary electrochemical reactions are adiabatic radiationless process, an isoenergetic transfer process between a position of Fermi level of the electrode and a position of equal energy in the discharging/or charging atom or molecule at the electrode surface.In the so-called outer-sphere approximation, the electron motion between its initial and final states was considered to be much more rapid than nuclear motions.Such an electronic motion, if considered a half-vibration, might take 10 -15 s, whereas the time required for a proton to move to a new position would be 10 -13 s, and 10 -11 s for a heavy molecule.Thus all heavy nuclei were imagined as frozen in space during the electron motion.Accordingly, according to the Marcus theory, the electron transfer during UOR on NiOOH is rapid process without electron accumulation to produce short-lived Ni 2+ to satisfy the Franck-Condon principle.That is, if the short-lived Ni 2+ can occur during UOR, the electron transfer will be not an adiabatic isoenergetic transfer process due to the different energy levels between Ni 2+ and Ni 3+ species.

Some secondary issues:
-Fig 4d .IR: C-N and C-O vibrations are almost baseline.Also, is there rationale why these vibrations are observed but no N-H ones?
-Sup.Fig. 13: the UOR equation is not equilibrated (O).I would at this pH write it with H2O, not H + .And thus the Nernst equation should also be corrected.
Reply: Thank you very much for your help.The chemical equation of anodic reaction of UOR is CO(NH2)2 + 6OH --6e -= N2 + CO2 + 5H2O.In theory, if the protons in the UOR are highly solvated to be released into the electrolyte, = 6 and = 6 .According to the Nernst equation, the equilibrium condition for this reaction under 25 o C and 1 atm is The pH dependence of UOR potential-current curves was recorded to understand the UOR mechanism (Supplementary Fig. 14a).To exclude the influence of background current and Ni(OH)2/NiOOH oxidation current, we obtain the initial UOR potential by the threshold current density method to define a potential at 10 mA cm -2 as UOR potential (V10 mA).Here, the V10 mA for UOR on Ni(OH)2 electrode is a linear function of pH with a negative slope of -54 mV/pH (Supplementary Fig. 14b), much close to -59 mV/pH slope of theoretical V10 mA versus pH for UOR with the completely solvated protons.Thus, the hydrogen of urea is released to electrolyte by the solvated reaction during UOR.
Supplementary Figure 14 | a, LSV curves for Ni(OH)2 electrode in 0.33 M urea-containing electrolyte with different pH values.To avoid the conductivity difference caused by the ion concentration difference, K + concentration was balanced to 1 M with the addition of K2SO4.b, The linear relationship of potential at 10 mA current vs. pH for Ni(OH)2 electrode.
-An "onset" potential has no meaning.One should better report the potential at a given current density.
Reply: Thank you very much for your help.Here, the initial UOR potential is defined as a potential to reach a current density of 10 mA cm -2 , that is, a threshold current density method (Curr.Opin.Electrochem. 2023, 37, 101176) to determine the initial UOR potential at which the current of a Faradaic process is measurable.
-The text is often very hard to read and understand.An important style-editing would be needed before publication.
Reply: Thank you very much for your help.We have rechecked and revised the sentences in whole text that are possible to make confusing.
In conclusion, the main claim of the authors does not appear to be backed enough by data and there is at this stage no solid experimental evidence that sustains the mechanistic hypothesis.In my opinion, the strongest claim that can currently be made is that Ni(II) are not long-lived intermediates (/resting states), which is however not surprising given the applied potentials.If the authors wish to further address the implication of the Ni(III) and Ni(II) species in UOR, I would suggest performing additional experiments, such a with operando spectroscopies that enable addressing oxidation state (XAS, XPS,…).On these grounds, I have selected reject and resubmit.Reply: Thank you very much for your help.We performed the in situ X-ray absorption near edge structure (XANES) measurements to monitor the Ni species during UOR as shown in Fig. 3d.The in situ XANES spectra revealed that the intensity of the white line decreases with polarizing the Ni(OH)2 electrode from 1.3 to 1.5 V in 1 M KOH + 0.33 M urea, suggesting the formation of higher-valence Ni species due to that the high-valence Ni inducing distortion in the NiO6 octahedral configuration will result in a decrease in the white line intensity (J.Electrochem.Soc. 1990,137, 383-388).Meanwhile, the edge energy (measured at half height) for the electrode at 1.3 and 1.5 V is the same as those of the Ni(OH)2 (8342.6 eV) and NiOOH (8344.5 eV), respectively.This edge position suggests a +3 nickel oxidation state for the electrode during UOR (J.Am.Chem. Soc. 2012, 134, 6801-6809).The in situ XANES results confirmed that during UOR the Ni 3+ is UOR-active and is likely to keep a constant valence state, in good agreement with the results from in situ Raman and in situ UV-vis absorption.In addition, in principle, as described in the mentioned-above discussions, we believe that the applied potentials polarize the Ni(OH)2 to form NiOOH, and then NiOOH is the stable working state to isoenergetically transfer electrons at the UOR elementary reaction steps.As described by Marcus theory, for an electrochemical process with a steady current flowing through the electrode, the isoenergetic tunneling of electrons (adiabatic radiationless process) has been proved to occur when reducing and oxidizing ions or molecules satisfy the requirement of the Franck-Condon principle.At each elementary reaction step, the active centers of the electrode are expected to exhibit the stable architecture for creating isoenergetic massive electron transfer.Accordingly, according to the Marcus theory, the electron transfer during UOR on NiOOH is rapid process without electron accumulation to produce short-lived Ni 2+ to satisfy the Franck-Condon principle.That is, if the short-lived Ni 2+ can occur during UOR, the electron transfer will be not an adiabatic isoenergetic transfer process due to the different energy levels between Ni 2+ and Ni 3+ species.Indeed, the in situ Raman, in situ UV-vis absorption, in situ XAFS, and in situ EIS have demonstrated that the electron transfer is via Ni 3+ without visible changes in oxidation valence.The authors have addressed all my comments satisfactorily and I recommend acceptance of this work in Nature Communications.
Reviewer #4 (Remarks to the Author): In the revision of their work, Yan S. and coll.addressed most of the reviewers' comments although in a partially satisfactory manner.
In particular, I still do not find convincing evidence ruling out the occurrence of Ni(II) and I find the statement too strong for the proofs provided.I though recognize that the authors have tamed down the statement now referring to Ni(OH)2/NiOOH in the text.
Regarding electrolysis: do the authors observe CO2/carbonates in 13C NMR? N2 is an usual contaminant in headspace GC and one should be extremely careful when quantifying this gas.
It is even more questioning that the authors state: "The selectivity and faradaic efficiency of the gaseous product of UOR in N2-saturated electrolyte containing 1 M KOH + 0.33 M urea were analyzed by gas chromatography (GC, 8860, Agilent, USA)."I do not understand how the quantification of N2 product in a N2-saturated electrolyte is made.

Fig. 1 |
Fig. 1 | The challenges in understanding UOR and OER on Ni(OH)2 electrode.c, A scheme to describe our strategy to discover the electrooxidation mechanism of organics on Ni 3+ active center with a nonredox electron transfer.

Fig. 3 |
Fig. 3 | Evidences to trivalent nickel as active species for urea electrooxidation.d, Normalized in situ Ni K-edge XANES spectra for polarizing Ni(OH)2 electrode at different potentials in 1 M KOH + 0.33 M urea, referenced to Ni foil and NiOOH.

Fig. 5 |
Fig. 5 | The catalytic mechanism of UOR on Ni 3+ .d, Concentration of oxygen vacancies derived by quantitative EPR analysis for the as-prepared Ni(OH)2 electrode, the Ni(OH)2 electrode after UOR, and the Ni(OH)2 electrode after OER with urea soaking.

Fig. 3 |
Fig. 3 | Evidences to trivalent nickel as active species for urea electrooxidation.d, Normalized in situ Ni K-edge XANES spectra for polarizing Ni(OH)2 electrode at different potentials in 1 M KOH + 0.33 M urea, referenced to Ni foil and NiOOH.
Photon energy (eV) -The experiments with chelation by a DMG salt are not conclusive.Under catalytic conditions, the potential applied is always more positive than E(Ni(III/II)) and thus formed Ni(II) species short-lived.When CV cycling, it is not the case, since potential excursions below E(Ni(III/II)) quantitatively convert the species into Ni(II) over extended time.At most, these comparative experiments allow to conclude that there is no long-lived soluble Ni(II) intermediate in UOR and are by far not a "solid evidence [] that urea oxidation does not undergo the Ni 2+ /Ni 3+ redox couple".